The use of robot arms for positioning and placing objects is well known. Generally, the arms have Z, r and .theta. motion in a conventional cylindrical coordinate system. The r or straight line movement of the end effector or mechanical hand at the end of the arm has been accomplished in a number of manners. For example, telescoping arms have been utilized for this purpose. However, these can cause a problem in generating particles which can interfere with processing of articles such as semiconductor wafers.
United Kingdom Patent Application GB 2193482A, published Feb. 10, 1988 and claiming priority from U.S. patent application Ser. No. 856,749 filed Aug. 5, 1986 discloses a wafer handling arm which includes two unequal length links with the distal end of one link being pivotally attached to the proximal end of the other link, with the hand being integral with the distal end of the distal link and which utilizes a belt drive which is fixed to a cam to attain nearly straight line motion. The accuracy of the motion is inherently sensitive to the cam shape and placement. Also, the pivots are exposed whereby particles can be produced at the pivots and at the cam.
It is also known to utilize an isosceles triangle type linkage wherein two equal length links are pivoted together and a mechanical hand is pivoted to the distal end of the distal link. Pulleys and belts are utilized in such a manner that the angle between the two links changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. This linkage provides drive directly from a motor shaft to the proximal end portion of the proximal link. A belt about a stationary pulley coaxial with the motor shaft passes about a pulley at the point of pivoting of the two links to one another. Another pulley and belt arrangement provides pivoting of another pulley where the second link is pivotally connected to the mechanical hand. There are certain disadvantages in such a system. First, the transmission of force from the motor via pivoting of the first link to the pivot which controls the mechanical hand is through the intermediate pivot where rigidity is not present and the length of transmission is sufficiently far so that the positioning of the hand is not nearly so accurate as would be desirable. Next, any chattering or cogging of the motor is transmitted directly or even multiplied at each of the pulleys and thereby at each of the pivot points of the apparatus.
In another apparatus a pair of isoceles triangle type linkages face one another and the mechanical hand is pivotally attached to the distal ends of both of the distal links. The proximal ends of each of the proximal links is driven in an opposite direction of rotation by a single rotating motor shaft, generally through use of appropriate gearing. What results is a frogs leg type of motion with each isoceles triangle type linkage serving as means for controlling the other such linkage in such a manner that the angles between the two links of each of the isosceles triangle linkages changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. Exposed metal gears can lead to particle formation. And, like the previously discussed isosceles triangle type linkage, the drive is at the proximal end of the proximal links. As a result, the mechanical hand, due to weight considerations, slack in pivots, etc., cannot be as accurately placed as would be desirable. Also, any chattering or cogging of the motor is transmitted directly via the gearing to the proximal end portions of the proximal links.
The present invention is directed to overcoming one or more of the problems as set forth above.